This invention relates to a circuit for controlling a free-piston engine, in particular of a refrigerator compressor.
As is well known, a free-piston engine of a compressor has to be fed with alternating (sinusoidal) current of variable amplitude and frequency.
It is also known that the power generated by the engine as a function of parameters, such as the temperature and humidity of the environment in which the refrigerator is located, has to be varied as said parameters vary.
Generally, the amplitude of the feed current is fixed for certain particular values of said parameters, whereas the frequency of this current is varied, by discrete values, until the frequency at which the mains power absorbed by the engine and therefore the power generated by the engine are a maximum.
Various circuits have already been proposed for monitoring and controlling the feed to an engine of the said type, which in particular enable the frequency of said feed to be varied in accordance with the variation in the environmental parameters of the place in which the refrigerator is located.
A known control circuit of the aforesaid type is described in EP 0 266 835 and comprises a circuit part of H configuration (or H bridge) to which the free-piston engine is connected and from which a current having a value proportional to the current absorbed by the engine is led off. At the same time, a voltage signal taken from a parallel-connected transformer is fed to a multiplier, for example, of the four-quadrant type, to which the said current signal is also fed. An analogous current signal is fed to a differential amplifier.
The multiplier provides an output an signal representing the product of the current and voltage signals, this signal therefore representing the power generated by the engine.
This power signal, after suitable filtration, is fed to a microprocessor which, using a, preset program, evaluates whether the determined power is the optimum or not.
Based on this determination, the microprocessor generates a current signal which is used a reference in the differential amplifier.
This latter provides an output a signal which is used to control the opening and closure of electronic switches connected into the sides of the H bridge, in order to allow passage of the engine feed current across the diagonals of these sides.
The known circuits have various drawbacks.
In particular, circuits using multipliers do not have high operational reliability and are often subject to faults, the repair of which involves high cost.
In addition, such circuits are costly to construct, reflecting the cost of the components used.